The present invention is directed generally to a system and method for controlling brake actuation energy in an electronically controlled vehicle brake system (xe2x80x9cEBSxe2x80x9d).
A conventional EBS is designed to control brake lining wear and the adhesion relationship (that is, the coefficient of friction) between the vehicle wheels and the road surface. This is accomplished conventionally by controlling brake lining wear in the light braking range and adhesion in the heavy braking range.
When a vehicle driver depresses the brake pedal, the output brake actuation energy, in an initial range up to a presettable driver""s set brake signal, is calculated solely according to criteria for controlling brake lining wear. In this initial range, axle loads are not considered. If the driver""s set brake signal is reached, the control system switches from the light braking (brake lining wear) control range to the heavy braking (adhesion) control range. In the heavy braking range, comfort and safety aspects outweigh brake lining wear considerations.
The control objective in the heavy braking adhesion control range is equal adhesionxe2x80x94to achieve equal slip during braking for all vehicle wheels with no differential slip between vehicle axles. Axle loads are taken into consideration for calculation and control of brake actuation energy as the kinetic energy to be converted in the brakes is no longer equal from one axle to another. If set differential slip thresholds are reached before the driver""s set brake signal is reached, the switch from the light braking brake lining wear control range to the heavy braking adhesion control range is triggered.
When, for example, a bus full of passengers is traveling downhill, the bus driver typically has to brake frequently, with the result that brake temperatures increase relatively quickly. As the brakes becomes hotter, the coefficient of friction (xcexc) of the friction pair of the brake (brake lining/brake disk or brake lining/brake drum) decreases, sometimes by as much as 100%. A decrease in the coefficient of friction of the friction pair translates into poorer braking power.
If the brakes of the vehicle""s front and rear axles heat to different temperatures, as is usually the case, the reduction of braking power is greatest where the temperature is highest. Under such conditions, the differential slip deviates from normal. Such a differential slip is compensated for by the EBSxe2x80x94the control objective of which is a differential slip of zero.
In commercial motor vehicles, especially trucks or buses, the cooling conditions for rear axle brakes are typically poorer than those for front axle brakes. Moreover, rear axle loads are routinely about 50 to 100% greater than front axle loads. This means that the rear axle brakes become hotter than the front axle brakesxe2x80x94and this can occur even in the light braking brake lining wear control range.
Since the rear axle brakes are hotter than the front axle brakes, they generate less braking power at the same brake actuation energy. By virtue of the zero differential slip control objective of the EBS, more brake actuation energy is introduced into the rear axle brakes under EBS control. As a result, the rear axle brakes become even hotter, presenting a possible overheating situation and associated undesired reduction in braking power.
Accordingly, it is desired to provide an improved system and method for controlling brake actuation energy that compensate for the zero differential slip control objective of the EBS when appropriate to prevent overheating of vehicle wheel brakes during braking and undesired reduction in braking power.
Generally speaking, in accordance with the present invention, an improved system and method for effecting temperature dependent control of brake actuation energy in a vehicle EBS are provided that compensate for the zero differential slip control objective of the EBS when appropriate to prevent overheating of vehicle wheel brakes during braking and undesired reduction in braking power.
In addition to conventional light braking (brake lining wear) control and heavy braking (adhesion) control, wheel brake temperature control is provided in the system and method according to the present invention, such that, if wheel brake temperatures reach a preset limit value during braking, the distribution of brake actuation energy is shifted from wheel brakes having higher temperatures to wheel brakes having lower temperatures. This can be accomplished by introducing more brake actuation energy into the wheel brakes having lower temperatures and less brake actuation energy (i.e., than before the limit value was reached) into the wheel brakes having higher temperatures, which can have a neutral effect on total braking power or, if necessary, cause a reduction in total braking power. In such case, the differential slip is not corrected to zero; rather, a non-zero differential slip is permitted.
According to the present invention, the differential slip threshold for the light braking brake lining wear control range and/or the driver""s set brake signal for switching to the heavy braking adhesion control range is/are raised. Higher wheel brake temperatures mean a later transition to the adhesion control range, with commensurate greater protection of heated wheel brakes. The transition to the adhesion control range need not occur at a presettable higher driver""s set brake signal, but can occur at a sliding value determined by wheel brake temperature (i.e., transition value=f (wheel brake temperature)).
According to another embodiment of the present invention, a separate, additional energy distribution calculation using rear axle brake temperatures and front axle brake temperatures as variables, possibly together with wheel brake temperatures of further axles, can be made.
In a further embodiment of the present invention, a signal warning the vehicle driver of an overheating condition during braking and the possibility of undesired reduction in braking power can be generated, for example, by brake pedal feel or by a visual or audible indicator.
Accordingly, it is an object of the present invention to provide a system and method for controlling brake actuation energy in a vehicle EBS that overcome disadvantages associated with prior art systems and methods.
It is a further object of the present invention to provide an improved system and method for effecting temperature dependent control of brake actuation energy in a vehicle EBS that compensate for the zero differential slip control objective of the EBS when appropriate to prevent overheating of vehicle wheel brakes during braking and undesired reduction in braking power.
Still other objects and advantages of the present invention will in part be obvious and will in part be apparent from the specification.
The present invention accordingly comprises the various steps and the relation of one or more of such steps with respect to each of the others, and embodies features of construction, combinations of elements, and arrangement of parts which are adapted to effect such steps, all as exemplified in the following detailed disclosure, and the scope of the invention will be indicted in the claims.